Uh, It's not that simple. I'm sure they plenty of discussion went into that. With that being said, I'd hope that you're aware that GTA V was well in development years ago. Have you seen a tally of how much was spent on GTA V development? It is a substantial figure in the millions, and was started long, long ago. It's not like this is a indie project from 6 months ago with a concrete release date...so it's not like MS or Sony could have flipped a switch 2 or 3 years ago and came up with a bright idea to have it as a launch release. Additionally, you're forgetting that GTA V was delayed - it was originally supposed to be released early this year. So that would conflict with the launch of the XB1 and PS4.
I think, if anything, that MS or Sony would be wise to seek a new IP as a launch title. Think about it. Anything that is multiplatform to include 360 and PS3 would defeat the purpose of a launch title, which is to show off the abilities of the console. A multiplatform game cannot do that, as they are designed for the lowest common denominator - as you mentioned, it would be purely for framerate and nothing more. A better idea is to come up with something new and fresh which can actually show the hardware off, and that is exactly what MS and Sony are doing. They both have compelling (and great looking) launch titles planned. Titan's Fall and Kill Zone look VERY nice.